Re: [PATCH v13 14/20] KVM: x86: Enable guest_memfd mmap for default VM type

[David Hildenbrand <david@redhat.com>]

On 09.07.25 12:59, Fuad Tabba wrote:
> Enable host userspace mmap support for guest_memfd-backed memory when
> running KVM with the KVM_X86_DEFAULT_VM type:
> 
> * Define kvm_arch_supports_gmem_mmap() for KVM_X86_DEFAULT_VM: Introduce
>    the architecture-specific kvm_arch_supports_gmem_mmap() macro,
>    specifically enabling mmap support for KVM_X86_DEFAULT_VM instances.
>    This macro, gated by CONFIG_KVM_GMEM_SUPPORTS_MMAP, ensures that only
>    the default VM type can leverage guest_memfd mmap functionality on
>    x86. This explicit enablement prevents CoCo VMs, which use guest_memfd
>    primarily for private memory and rely on hardware-enforced privacy,
>    from accidentally exposing guest memory via host userspace mappings.
> 
> * Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in KVM_X86: Enable the
>    CONFIG_KVM_GMEM_SUPPORTS_MMAP Kconfig option when KVM_X86 is selected.
>    This ensures that the necessary code for guest_memfd mmap support
>    (introduced earlier) is compiled into the kernel for x86. This Kconfig
>    option acts as a system-wide gate for the guest_memfd mmap capability.
>    It implicitly enables CONFIG_KVM_GMEM, making guest_memfd available,
>    and then layers the mmap capability on top specifically for the
>    default VM.
> 
> These changes make guest_memfd a more versatile memory backing for
> standard KVM guests, allowing VMMs to use a unified guest_memfd model
> for both private (CoCo) and non-private (default) VMs. This is a
> prerequisite for use cases such as running Firecracker guests entirely
> backed by guest_memfd and implementing direct map removal for non-CoCo
> VMs.
> 
> Co-developed-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   arch/x86/include/asm/kvm_host.h | 9 +++++++++
>   arch/x86/kvm/Kconfig            | 1 +
>   arch/x86/kvm/x86.c              | 3 ++-
>   3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4c764faa12f3..4c89feaa1910 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2273,9 +2273,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>   #ifdef CONFIG_KVM_GMEM
>   #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
>   #define kvm_arch_supports_gmem(kvm)  ((kvm)->arch.supports_gmem)
> +
> +/*
> + * CoCo VMs with hardware support that use guest_memfd only for backing private
> + * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled.
> + */
> +#define kvm_arch_supports_gmem_mmap(kvm)		\
> +	(IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) &&	\
> +	 (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM)
>   #else
>   #define kvm_arch_has_private_mem(kvm) false
>   #define kvm_arch_supports_gmem(kvm) false
> +#define kvm_arch_supports_gmem_mmap(kvm) false
>   #endif
>   
>   #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index df1fdbb4024b..239637b663dc 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -47,6 +47,7 @@ config KVM_X86
>   	select KVM_GENERIC_HARDWARE_ENABLING
>   	select KVM_GENERIC_PRE_FAULT_MEMORY
>   	select KVM_GENERIC_GMEM_POPULATE if KVM_SW_PROTECTED_VM
> +	select KVM_GMEM_SUPPORTS_MMAP
>   	select KVM_WERROR if WERROR

Given the error, likely we want to limit to 64BIT.

select KVM_GMEM_SUPPORTS_MMAP if X86_64

-- 
Cheers,

David / dhildenb